The present invention relates generally to operations performed in subterranean wells and, in an embodiment described herein, more particularly provides a firing head for use in detonating an explosive device in a well.
There is a great potential for damage to a well and injury to personnel in operations which involve detonating an explosive device in the well. Such operations may include perforating protective casing lining the wellbore, severing tubing using an explosive tubing cutter, etc. In these operations, an explosive device and an apparatus for detonating the explosive device are typically conveyed into the well interconnected to each other. In the case of perforating operations, the explosive device is one or more perforating guns and the apparatus used to detonate the gun or guns is known as a firing head.
It is well known in the art to provide a variety of different methods of actuating the firing head. For example, the firing head may be actuated by dropping a bar onto the firing head through tubing extending from the firing head to a wellhead or rig floor at the earth's surface. The bar eventually strikes a firing pin in the firing head, which causes detonation of a detonator explosively coupled to the perforating gun. As another example, fluid pressure may be applied to the tubing, and/or to an annulus surrounding the tubing, to actuate the firing head. As yet another example, electric current may be applied to a blasting cap in the firing head, such as via a wireline extending to the earth's surface.
It is also well known in the art to provide a delay mechanism for delaying the detonation after the firing head has been actuated. This delay permits wireline, slickline, or other equipment to be removed from the well before the perforating guns are detonated, permits a favorable fluid pressure condition to be established before the perforating guns are detonated, etc. Unfortunately, such delay mechanisms sometimes impose an undesirable level of uncertainty into the operation, are at times complex and unreliable, and may not be stopped after the delay mechanism has been actuated.
For example, in a conventional pressure actuated firing head having a hydraulic delay, the firing head is attached to a perforating gun and conveyed into a well. When properly positioned therein, fluid pressure is applied to the firing head to shear one or more shear pins and begin the delay period. A piston in the delay mechanism forces hydraulic fluid through an orifice to produce the delay period, and, when the piston has traveled a predetermined distance, a port is opened, thereby permitting fluid pressure to enter a chamber and drive a firing pin to impact a detonator. In other pressure actuated firing heads, shearing of the shear pins causes initiation of a pyrotechnic time delay which, when it has completed burning, causes detonation of a detonator.
It will be appreciated that several problems exist with these types of firing heads, and other firing heads having delay mechanisms. First, it is difficult to determine from the earth's surface whether the firing head has been actuated at all. Thus, if the perforating gun does not detonate when expected, personnel at the earth's surface will be unsure whether the firing head has been actuated, and whether the firing head and perforating gun may be safely retrieved from the well. If the firing head has been actuated, but a problem has occurred with the delay mechanism, such as a piston binding in a bore, the perforating gun may detonate at any time, possibly causing extensive damage to the well and injury to personnel.
Second, several factors tend to influence typical delay mechanisms, making it difficult to determine the exact delay period to be expected after the firing head has been actuated. For example, the temperature in the well (which may be known only approximately) will influence the speed at which a piston is able to force fluid through an orifice, and will influence the burning rate of a fuse. As another example, the fluid pressure at which the shear pins shear, the fluid pressure applied to shear the pins and the fluid pressure used to drive a piston in a delay mechanism, may not be precisely known. Each of these may influence the initiation of the firing head and start of the delay period.
Third, typical firing heads with delay mechanisms have no provision for retrieving the firing heads to the earth's surface without a perforating gun attached thereto. It would greatly enhance the safety of operations if the delay mechanism could be positively stopped, and if the firing head could be retrieved to the earth's surface separately from the perforating gun.
Of course, the above problems may also apply to other operations involving detonation of an explosive device in a well. Therefore, a solution to these problems would be applicable not only to perforating operations, but to a variety of other operations as well.
From the foregoing, it can be seen that it would be quite desirable to provide an apparatus which does not utilize a complex or unreliable delay mechanism, and which does not introduce unacceptable levels of uncertainty and hazards, but which permits reliable, convenient and efficient detonation of an explosive device in a well. It is accordingly an object of the present invention to provide such apparatus and associated methods of detonating an explosive device.